feat(llm): structured error taxonomy with a shared HarnessError base (RFC 005 pt 2)

Introduce HarnessError in dsh-llm (the leaf package): a stable machine-routable
code distinct from the message, cause chaining, name from the subclass, plus
isHarnessError. LlmError, ToolArgsError, and InvariantError now extend it.

Tool failures carry the structure end-to-end: ToolExecutionResult gains
error: { name, code } (populated from a thrown HarnessError), and the loop
forwards it onto the tool/result session event (which gained the same optional
field) for retry/sandbox plugins and replay. The loop's toError wraps non-Error
throws in a HarnessError(code: UNKNOWN, cause) instead of a bare Error.

Landed last and in isolation so it's a pure upgrade over the plain Error+code
the earlier PRs used — independently revertible. Graduates RFC 005 pt 2 ->
ADR 0015; RFC 005 now fully implemented.

packages/tools/src/schema.ts

@@ -20,7 +20,7 @@
  */
 
 import type { ContentBlock } from '@deepseek-ai/dsh-llm'
-import { assertNever } from '@deepseek-ai/dsh-llm'
+import { assertNever, HarnessError } from '@deepseek-ai/dsh-llm'
 import type { ToolDefinition, ToolExecution } from './index.ts'
 
 // ---------------------------------------------------------------------------
@@ -174,20 +174,17 @@ export function schemaSpecToJsonSchema(spec: SchemaSpec): JsonSchemaObject {
 
 /**
  * Thrown by a {@link defineTool} tool when the model-generated arguments don't
- * match the declared {@link SchemaSpec}. The registry's execute waterfall
- * catches it and returns an `isError` result so the model can self-correct.
- *
- * Plain `Error` for now (carries a `code` field); a later change promotes the
- * harness error taxonomy and this extends a common base.
+ * match the declared {@link SchemaSpec}. Extends {@link HarnessError}
+ * (`code: 'INVALID_ARGS'`); the registry's execute waterfall catches it and
+ * returns an `isError` ToolExecutionResult carrying the structured error, so
+ * the model can self-correct and downstream plugins can route on the code.
  */
-export class ToolArgsError extends Error {
-  /** Machine-routable code; stable across the message wording. */
-  readonly code = 'INVALID_ARGS'
+export class ToolArgsError extends HarnessError {
   /** The individual violation messages, in declaration order. */
   readonly violations: string[]
 
   constructor(violations: string[]) {
-    super(`invalid arguments: ${violations.join('; ')}`)
+    super(`invalid arguments: ${violations.join('; ')}`, 'INVALID_ARGS')
     this.name = 'ToolArgsError'
     this.violations = violations
   }